gh-150942: Speed up csv.reader row building

Author: omkar-334

Misc/NEWS.d/next/Library/2026-06-05-12-00-00.gh-issue-150942.Hkdh87.rst

@@ -0,0 +1,3 @@
+Speed up :func:`csv.reader` by appending parsed fields to the output row
+without an extra reference-count round-trip (using the internal
+reference-stealing list append helper).

Modules/_csv.c

@@ -14,6 +14,7 @@ module instead.
 #endif
 
 #include "Python.h"
+#include "pycore_list.h"          // _PyList_AppendTakeRef()
 #include "pycore_pyatomic_ft_wrappers.h"
 
 #include <stddef.h>               // offsetof()
@@ -685,11 +686,9 @@ parse_save_field(ReaderObj *self)
         }
         self->field_len = 0;
     }
-    if (PyList_Append(self->fields, field) < 0) {
-        Py_DECREF(field);
+    if (_PyList_AppendTakeRef((PyListObject *)self->fields, field) < 0) {
         return -1;
     }
-    Py_DECREF(field);
     return 0;
 }